Greenhouse effect could extend habitable zone

August 26, 2011
By Nola Taylor Redd

The habitable zone of a star is the orbital area where water can be a liquid on a planet’s surface. Based on temperature, the habitable zone differs from star to star. Hotter stars have habitable zones farther out than cooler stars. Credit: NASA/Kepler Mission

The distant region beyond Saturn is too cold for liquid water, a necessity for life as we know it. But new research indicates that rocky planets far from their parent star could generate enough heat to keep water flowing - if their atmospheres were made up primarily of hydrogen.

Planets near their suns reap the benefits of light and heat, while those farther away must endure colder temperatures. But the new research indicates that planets with hydrogen-rich atmospheres could contain liquid at their surface even out to fifteen times the distance between the Earth and the Sun.

With a hydrogen atmosphere, the greenhouse effect these planets could experience would be sufficient to allow for liquid water on their surfaces, despite their distant orbits.

The area around a star in which water can be liquid rather than ice is known as the habitable zone. Sometimes called the "Goldilocks zone," it's just right - not too hot (so the water doesn't evaporate) and not too cold (so it wont freeze).

Typically, the distance calculated takes into account a rocky body having an atmosphere made up of water and carbon dioxide, the same greenhouse gases found on Earth.

"This is the kind of planet we know is habitable," explains Raymond Pierrehumbert of the University of Chicago, lead author on a paper published recently in the Astrophysical Journal Letters.

Although life could feasibly exist in many locations throughout the universe, we only know of one place it definitely formed - Earth. Thus astronomers find themselves seeking other, similar planets in the hopes of finding life elsewhere.

This artist's image shows an icy/rocky planet orbiting a red dwarf, a relatively cool star. If such a planet maintained a hydrogen atmosphere, it could conceivably hold liquid water on its surface. Credit: NASA, ESA and G. Bacon (STScI)

The size of a solar systems habitable zone varies, however, depending on the properties of the star. For hotter, brighter stars, the region stretches farther out into space, while its inner edge cannot be too close to the star.

The habitable zone for a G-type star such as the Sun, for instance, lies between 0.95 and 1.4 AUs (one AU, or astronomical unit, is the distance from the Earth to the Sun). The Earth, quite obviously, falls within that region. For a smaller, dimmer M-type star, the habitable zone is closer, between 0.08 and 0.12 AUs.

But according to Pierrehumbert's research, , a rocky planet with a hydrogen atmosphere could have a habitable zone extending as far as 1.5 AUs for M-stars and 15 AUs for G-stars.

This means that for stars similar to the Sun, rocky planets beyond the reach of Saturn could contain oceans of water.

Travis Barman of Lowell Observatory notes that there might be many similar scenarios of planets that don't mimic Earth but are still habitable. Barman, who was not part of the new research, has done extensive research of his own modeling the atmospheres of extrasolar planets.

Speaking of the new research, Barman said, "it broadens our notion of the habitable zone, at least in the sense of basic life Such work is timely, as the search for potentially habitable terrestrial planets is just ramping up."

Pierrehumberts team had to locate a "Goldilocks zone" for hydrogen-rich planets orbiting different types of stars. If such a planet were too close to its parent star, the stellar energy could destroy the hydrogen atmosphere. But more distance reduced the chance of liquid water on the planets surface.

"Happily, there appears to be a zone close enough to allow liquid water, but not so close as to cause the atmosphere to be lost," said Pierrehumbert.

Then, too, the formation of such an atmosphere doesn't mean it will be around long enough for life to develop.

Pierrehumbert points out that the tectonic activity such as volcanism or earthquakes could release carbon monoxide, converting the hydrogen into methane.

At the same time, microbial life could consume hydrogen, destroying the atmosphere that supports them.

"Making the whole system work, especially on an M-star, is quite tricky," he said. "You may have a lot of failed habitable planets out there because they didn't find a sustaining process."

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20 comments

"liquid water, a necessity for life as we know it" this right here is wrong, as we know life has been discovered living in arsenic and they are totally different to us there need not be water for life to arise/exist

"liquid water, a necessity for life as we know it" this right here is wrong, as we know life has been discovered living in arsenic and they are totally different to us there need not be water for life to arise/exist

Life has not been discovered that lives on Arsenic instead of water! There was a NASA report stating that a bacterium was discovered that used As instead of P, but even that has been debunked. Life AS WE KNOW IT requires liquid water.

Bowler, IIRC, those bacteria used *some* arsenic to replace phosphorous in *some* parts of their metabolism, but the rest of their chemistry was not unusual. The high tolerance of arsenic they'd developed let them grab a toxic niche free of potential rivals. Generation by generation, mutation by mutation, they'd moved up the headwaters of that toxic stream....

...Am I the only one completely lost by the whole business of hydrogen serving as a greenhouse gas? It has no dipole moment, and it's so small that it couldn't even manage to muster up much of a temporary dipole. How is it possible that it can absorb infrared? Surely not some odd low-energy electronic transition; in a thick atmosphere, none of the upper orbitals that'd be needed for such a transition could possibly remain populated, and H2 itself is so small that the molecular orbital for it is going to have transitions that are in the UV range or higher energy. What am I missing, here?

@Ronan - I'm not understanding your confusion. Hydrogen has a red/infrared absorbtion spectrum. So it would capture heat very efficiently.

For life to live in this range of planet, you would virtually require higher life to use Hy instead of O. (An atmosphere can have an abundance of one or the other, but not both. They are both very reactive elements, and will create water until there is only one left.)

If this is true, why isn't Jupiter and Saturn hot enough to have liquid water?

Jupiter is 82% hydrogen and 12% helium.

Jupiter and Saturn are TOO hot for liquid water. Even Uranus and Neptune are marginally too hot. Below the 200 bar pressure level, the temperature passes the critical point of water, so it never condenses. Their internal warmth is kept in too efficiently for oceans to form.

"If such a planet were too close to its parent star, the stellar energy could destroy the hydrogen atmosphere."

What??

Hydrogen escapes too easy thanks to the solar wind and heating of the exosphere of the planet. Also UV busts it into atomic hydrogen which then escapes thermally. Past a certain distance and the loss rate drops dramatically - about 2.5 AU for a sun-like star.

"liquid water, a necessity for life as we know it" this right here is wrong, as we know life has been discovered living in arsenic and they are totally different to us there need not be water for life to arise/exist

Life has not been discovered that lives on Arsenic instead of water! There was a NASA report stating that a bacterium was discovered that used As instead of P, but even that has been debunked. Life AS WE KNOW IT requires liquid water.

I've done the math and with ONLY six HARD variables there are about a quarter of a million stars in this galaxy which could host or evolve life as we know it. With the blatant assumptions that people make on the other side of the argument the number is less than one for the observable universe. The Drake equation is OUTDATED. If you're REALLY interested PM me and I'll give you the links and the sources.

I've always been a sci-fi fan. I WANT there to be a million civilizations out there for us to interact with and such, but it won't be the case. Earth is very, very, VERY rare. Any protests to the contrary will be put to rest by Kepler.

Jupiter and Saturn are TOO hot for liquid water. Even Uranus and Neptune are marginally too hot. Below the 200 bar pressure level, the temperature passes the critical point of water, so it never condenses. Their internal warmth is kept in too efficiently for oceans to form

Jupiter actually radiates more energy than it receives from the Sun, suggesting relatively little greenhouse effect. The planet gradually shrinks through accretion as the heat escapes, bringing the atmosphere to lower and lower energy levels.

Temperature and pressure are related, so naturally, the center of a gas giant is going to be super-heated for a very long time, even if they were like way out in a star's kuiper belt.

I've always been a sci-fi fan. I WANT there to be a million civilizations out there for us to interact with and such, but it won't be the case. Earth is very, very, VERY rare. Any protests to the contrary will be put to rest by Kepler.

I don't think interacting with other species, if they do exist, would be wise.

Nature shows us that all life does is destroy other life. Almost everything in nature exists by killing something else.

In fact, all animals, including herbivores, exist by destroying other life.

the expectation that you would meet some Vulcans, or something like them, and have a tea party with them is almost certainly flawed.

We can't even have peace with our own species, and you think we'd be friends with some aliens? Hardly.

Either they'd kill us, or we'd kill them, the same as much of anything else in nature.

If, by hydrogen-rich, the article means that a large proportion of the atmosphere of such a rocky planet was composed of molecules of diatomic hydrogen, then I must pose the same question as Ronan above : my understanding is that such homonuclear molecules have no net change in their dipole moment when they vibrate and thus are almost totally unaffected by infrared light. Furthermore, such a rocky planet would have to be extremely massive to hold a H2 atmosphere, which otherwise would be blown away by the stellar wind....